A NEW APPROACH TO LANDSLIDE SUSCEPTIBILITY MAPPING USING INVENTORY MAPS: A CASE HISTORY OF NATURAL GAS PIPELINES IN NORTHWEST OREGON, USA

The study area runs along three natural gas transmission pipelines passing through ancient landslides, steep terrain, and weak bedrock geology within a rainfall zone that reaches 510 cm/yr. These natural gas pipelines cross 300 km of mostly Cenozoic basalts and marine sediments of the Oregon Coast Range with 269 landslides (148 field checked) mapped in a 1.6 km width corridor. 59 geologic units were mapped from ten compiled geologic maps.

The key discovery found in the study within the Coast Range is that 83% of all landslides originated at geological contacts which creates a new focus for determining landslide susceptibility. Landslides of lithologic contacts according to occurrence (O_k) and length coverage (C_k), area density within geologic units (G_k), and slope (theta) can be compiled into one function, T_i, which determines TIN triangle values for landslide susceptibility. The developed model was tested in the neighboring area of Astoria, Oregon with 77% of the landslides located on predetermined higher ranked TIN triangles.

Each pipeline has their own individual parameters for T_i which includes the following functions: landslide area density for each geologic unit (G_i ), lithologic units with percentage of landslides occurring on their contacts (O_k ), lithologic units with length percent running through landslides material (C_k ), and minimum slope angle (theta). Pipeline corridors susceptible to landslides are segments with basalts capping older and weaker marine sediments. Mean slope of failure for all three pipelines is 17^o (mode, 11^o, minimum, 5^o).